Myself, I think the warlock does better when he doesn't rely on the blast so much. The "at will" qualities of his invocations really see some use when he starts dispelling every buff or magic item that the enemies bring to bear, and laying down walls of fire to cook those caught in his chilling tentacles.*
The warlock works best when he stays back and just wrecks the battlefield, firing off the odd eldritch blast to catch stragglers, hindering and blocking, dispelling enemy buffs, and Fleeing the Scene when he gets charged by the enemy. He's not as effective as a regular archer, but then regular archers can't throw down area-effect mayhem or dispels. He's not as effective as a wizard, but he's not as susceptible to the "quick, kill the arcane caster before he roasts us" problem. He's also pretty handy with the wand of polymorph, staff of healing, or scroll of meteor swarm. Especially because he could have crafted any of them, if he takes an appropriate feat.
The high-level warlock seems to shine in breadth, rather than depth. It has those generalist weaknesses that come from lack of specialty, but with the item crafting and some well-planned invocation synergies, it makes a really good one-man support crew.
*Actually, that sounds like a really stylish combo. Tentacle a group of creatures and then put a ring of fire around it. Cold damage plus fire damage plus bludgeoning tentacles equals ouch. Then if you manage to break free of the tentacles, you still have to get through the wall. The lowest level you could pull this trick off is 13th.